Update travis build file : add --prefer-source to composer install, add build on PHP 5.5
Update travis build file : add --prefer-source to composer install, add build on PHP 5.5

alawnchen/PHP-FFMpeg,alawnchen/PHP-FFMpeg,gcc-roblox/gcc-roblox.github.io,burzum/PHP-FFMpeg,Pixy/PHP-FFMpeg,gcc-roblox/gcc-roblox.github.io,NAMEs/PHP-FFMpeg,mruse/PHP-FFMpeg,curtisdf/PHP-FFMpeg,lb0913/PHP-FFMpeg,soluciones-gbh/PHP-FFMpeg,beeseenmedia/PHP-FFMpeg,we-bridge/PHP-FFMpeg,superdol/PHP-FFMpeg